1. Field of the Invention
The present invention relates to a light-emitting gallium nitride-based compound semiconductor device and, more particularly, to a light-emitting compound semiconductor device having a double-heterostructure capable of emitting high-power visible light ranging from near-ultraviolet to red, as desired, by changing the composition of a compound semiconductor constituting an active layer (light-emitting layer).
2. Description of the Related Art
Gallium nitride-based compound semiconductors such as gallium nitride (GaN), gallium aluminum nitride (GaAlN), indium gallium nitride (InGaN), and indium aluminum gallium nitride (InAlGaN) have a direct band gap, and their band gaps change in the range of 1.95 eV to 6 eV. For this reason, these compound semiconductors are promising as materials for light-emitting devices such as a light-emitting diode and a laser diode.
For example, as a light-emitting device using a gallium nitride semiconductor, a blue light-emitting device in which a homojunction structure is formed on a substrate normally made of sapphire through an AlN buffer layer has been proposed. The homojunction structure includes a light-emitting layer formed of p-type impurity-doped GaN on an n-type GaN layer. As the p-type impurity doped in the light-emitting layer, magnesium or zinc is normally used. However, even when the p-type impurity is doped, the GaN crystal has a poor quality, and remains an i-type crystal having a high resistivity almost close to an insulator. That is, the conventional light-emitting device is substantially of a MIS structure. As a light-emitting device having the MIS structure, layered structures in which Si- and Zn-doped, i-type GaAlN layers (light-emitting layers) are formed on n-type CaAlN layers are disclosed in Jpn. Pat. Appln. KOKAI Publication Nos. 4-10665, 4-10666, and 4-10667.
However, in the light-emitting device having the MIS structure, both luminance and light-emitting output power are too low to be practical.
In addition, the light-emitting device of a homojunction is impractical because of the low power output by its nature. To obtain a practical light-emitting device having a large output power, it is required to realize a light-emitting device of a single-heterostructure, and more preferably, a double-heterostructure.
However, no light-emitting semiconductor devices of a double-heterostructure are known, in which the double-heterostructure is entirely formed of low-resistivity gallium nitride-based compound semiconductors, and at the same time, has a light-emitting layer consisting of low-resistivity, impurity-doped InGaN.
Jpn. Pat. Appln. KOKAI Publication Nos. 4-209577, 4-236477, and 4-236478 disclose a light-emitting device having a double-heterostructure in which an InGaN light-emitting layer is sandwiched between an n-type InGaAlN clad layer and a p-type InGaAlN clad layer. However, the light-emitting layer is not doped with an impurity, and it is not disclosed or explicitly suggested that an impurity is doped into the light-emitting layer. In addition, the p-type clad layer is a high-resistivity layer in fact. A similar structure is disclosed in Jpn. Pat. Appln. KOKAI Publication No. 64-17484.
Jpn. Pat. Appln. KOKAI Publication 4-213878 discloses a structure in which an undoped InGaAlN light-emitting layer is formed on an electrically conductive ZnO substrate, and a high-resistivity InGaN layer is formed thereon.
Jpn. Pat. Appln. KOKAI Publication No. 4-68579 discloses a double-heterostructure having a p-type GaInN clad layer formed on an oxygen-doped, n-type GaInN light-emitting layer. However, another clad layer consists of electrically conductive ZnO. The oxygen is doped in the light-emitting layer to be lattice-matched with the ZnO. The emission wavelength of the light-emitting device having this double-heterostructure is 365 to 406 nm.
All conventional light-emitting devices are unsatisfactory in both output power and luminance, and have no satisfactory luminosity.
It is an object of the present invention to provide a double-heterostructure in which all of the light-emitting layer (active layer) and the clad layers are formed of low-resistivity gallium nitride-based III-V Group compound semiconductors, thereby realizing a semiconductor device exhibiting an improved luminance and/or light-emitting output power.
It is another object of the present invention to provide a light-emitting device excellent in luminosity.
It is still another object of the present invention to provide an ultraviolet to red light-emitting device having a wavelength in the region of 365 to 620 nm.
According to the present invention, there is provided a light-emitting gallium nitride-based compound semiconductor device having a double-heterostructure comprising:
a light-emitting layer (active layer) having first and second major surfaces and formed of a low-resistivity InxGa1xe2x88x92xN (0 less than x less than 1) compound semiconductor doped with an impurity;
a first clad layer joined to the first major surface of the light-emitting layer and formed of an n-type gallium nitride-based compound semiconductor having a composition different from that of the compound semiconductor of the light-emitting layer; and
a second clad layer joined to the second major surface of the light-emitting layer and formed of a low-resistivity, p-type gallium nitride-based compound semiconductor having a composition different from that of the compound semiconductor of the light-emitting layer.
In the first embodiment, the compound semiconductor of the light-emitting layer (active layer) is of p-type, doped with a p-type impurity.
In the second embodiment, the compound semiconductor of the light-emitting layer (active layer) remains an n-type, doped with at least a p-type impurity.
In the third embodiment, the compound semiconductor of the light-emitting layer (active layer) is of n-type, doped with an n-type impurity.
In the present invention, the compound semiconductor of the first clad layer is preferably represented by the following formula:
GayAl1xe2x88x92yN(0xe2x89xa6yxe2x89xa61)
The compound semiconductor of the second clad layer is preferably represented by the following formula:
GazAl1xe2x88x92zN(0xe2x89xa6zxe2x89xa61)
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.